Method for Operating a Nitrogen Oxide Storage Catalytic Converter in a Motor Vechicle with Consideration of Regional Fuel Qualities

ABSTRACT

The fuel qualities for vehicles differ, for example, in Europe on a regional basis. This applies in particular to the sulphur content of the fuel. If vehicles with modern lean engines or with diesel engines which are equipped with nitrogen oxide storage-type catalytic converters in order to reduce the nitrogen oxide content in their exhaust gas pass through regions with a sulphur content in the fuel which is above the sulphur content which is specified for the vehicle, the engine controller will correspondingly frequently initiate desulphurization of the storage-type catalytic converter after the vehicle has been refuelled with this fuel. Any desulphurization entails an increased level of fuel consumption and greater ageing of the catalytic converter. It is proposed to avoid these adverse effects in that in lean motors the engine controller switches to stoichiometric operation in such a case, and in a diesel engine said engine controller prevents the desulphurization. The information about the region in which the vehicle is moving at a particular time can be transmitted to the engine controller by a navigation system.

The invention relates to a method for operating a nitrogen oxide storage-type catalytic converter on a motor vehicle taking into account regional fuel qualities.

Vehicles with lean motors or diesel engines are operated with lean air/fuel mixtures. In order to reduce the nitrogen oxide content in their exhaust gas, they are equipped with nitrogen oxide storage-type catalytic converters. Nitrogen oxide storage-type catalytic converters usually contain, as storage components, basic oxides of the alkali metals or alkaline earth metals such as, for example, barium oxide and strontium oxide and they store the nitrogen oxides contained in the exhaust gas in the form of nitrates of these metals. As a result, the storage capacity of the catalytic converters for the nitrogen oxides is continuously reduced. They must therefore be regenerated from time to time. This is done by briefly enriching the air/fuel mixture. While the normal operation of the vehicles with a lean air/fuel mixture lasts approximately 1 to 5 minutes depending on the nitrogen oxide content of the exhaust gas, the regeneration is respectively already terminated after a few seconds.

Under the conditions prevailing in the exhaust gas, the content of sulphur which is always contained in motor fuels forms sulphates with the storage components, as a result of which the storage capacity for the nitrogen oxides is reduced. The nitrogen oxide storage-type catalytic converters must therefore be desulphurized at relatively long or short time intervals. This is done by heating the catalytic converter to at least 600° C. and changing the engine operating mode from lean operation to stoichiometric or rich operation. The stoichiometric or rich operation of the engine is absolutely necessary here for the desulphurization to be already possible at approximately 600° C. Any desulphurization therefore results in an increased level of fuel consumption and contributes to early ageing of the storage-type catalytic converter. In the normal operating mode of the engine, that is to say under lean exhaust gas conditions, the temperature of the catalytic converter would, on the other hand, have to be increased to approximately 1200° C. in order to thermally decompose the sulphates.

In modern motor vehicles, the electronic engine controller is equipped with a diagnostic system (OBD=on board diagnostic) which automatically initiates desulphurization when a specific degree of sulphurization of the storage-type catalytic converter has been reached.

Because of the significance for satisfactory operation of the engine and for the correct functioning of the exhaust gas purification system, vehicle manufacturers generally prescribe the fuel quality which is to be used for the vehicle.

The fuel qualities for vehicles differ regionally in Europe. This applies in particular to the sulphur content of the fuel. The sulphur content of spark-ignition fuels and diesel fuels is usually below 50 ppm. However, in some regions of Europe fuels with relatively high sulphur contents are permitted. Similar conditions also apply on other continents.

Therefore, if a vehicle which is equipped with a nitrogen oxide storage-type catalytic converter on a lean engine or diesel engine travels from a region with a low sulphur content in the fuel to a region with a high sulphur content in the fuel, the electronic engine controller will initiate correspondingly more frequent desulphurization of the nitrogen oxide storage-type catalytic converter. This entails increased fuel consumption and more rapid ageing of the nitrogen oxide storage-type catalytic converter.

DE 101 58 480 C1 describes a method for operating an engine in a vehicle, and in this context navigation signals are read in from an external navigation system as driving state parameters in order, for example, to perform the desulphurization of a nitrogen oxide storage-type catalytic converter when travelling on a motorway.

Likewise, WO 01/25606 A1 describes a control unit for regenerating a storage-type catalytic converter which is connected to a navigation system. The navigation system uses the known route information as a basis for determining, before the start of the regeneration, the time and the location at which the storage-type catalytic converter can be regenerated without infringing local regulations for emissions.

EP 0 859 132 A1 also describes an emissions control unit which uses information about the driving state from a navigation system in the regeneration of, for example, a nitrogen oxide storage-type catalytic converter.

None of these patent documents indicates how the problems which are associated with the fuel qualities which differ on a regional basis can be solved. Correspondingly, the object of the present invention is to specify a method which can overcome the described disadvantages when vehicles pass through regions with different fuel qualities.

In order to achieve this object, the invention is based on a motor vehicle which is equipped with a petrol engine which operates in lean mode or with a diesel engine and has an electronic engine controller and a navigation system. The method is characterized in that the electronic engine controller switches to stoichiometric operation in the case of the petrol engine which operates in the lean mode or prevents the automatic desulphurization of the nitrogen oxide storage-type catalytic converter in the case of the diesel engine if the navigation system recognizes that the vehicle is moving in a region with a high sulphur content in the fuel.

According to the invention, the petrol engine which is operating in lean mode is switched to stoichiometric operation in regions with a high sulphur content in the fuel. In this case, the nitrogen oxide storage-type catalytic converter acts as a three-way catalytic converter. As has been stated above, the sulphates which are formed on the catalyst already decompose at exhaust gas temperatures above 600° C. under stoichiometric or rich exhaust gas conditions. These temperatures are generally reached on a frequent basis in a petrol engine which is operated in stoichiometric mode. In the case of a diesel engine, it is not possible to switch to stoichiometric operation permanently. For this reason, according to the invention the desulphurization of the storage-type catalytic converter is blocked by the engine controller at high sulphur contents which exceed the limiting value.

Preferably, switching to stoichiometric operation or the prevention of the automatic desulphurization is only initiated after the fuel which is still present in the fuel tank when the region with a high sulphur content has been entered has been consumed and it is necessary to refuel. Likewise, the engine controller preferably does not switch back to the normal operating mode until the vehicle enters a region with the fuel qualities which are prescribed for the vehicle and the fuel with a relatively high sulphur content has been consumed. Alternatively, the switching to stoichiometric operation or the prevention of the automatic desulphurization can also be initiated when the measurement of the nitrogen oxide concentration with, for example, a nitrogen oxide sensor downstream of the nitrogen oxide storage-type catalytic converter exceeds a specific threshold value. 

1. A method for operating a nitrogen oxide storage-type catalytic converter on a motor vehicle taking into account regional fuel qualities, wherein the vehicle is equipped with a petrol engine which operates in lean mode or with a diesel engine and has an electronic engine controller and a navigation system, characterized in that the electronic engine controller switches to stoichiometric operation in the case of the petrol engine which operates in the lean mode or prevents the automatic desulphurization of the nitrogen oxide storage-type catalytic converter in the case of the diesel engine if the navigation system recognizes that the vehicle is moving in a region with a high sulphur content in the fuel.
 2. The method according to claim 1, characterized in that the switching to stoichiometric operation or the prevention of the automatic desulphurization does not take place until the fuel which is still present in the fuel tank when the region with a high sulphur content has been entered has been consumed and it is necessary to refuel.
 3. The method according to claim 1, characterized in that the electronic engine controller is switched back to the normal operating mode again when the vehicle enters a region with the fuel quality which is prescribed for the vehicle, and the fuel with a relatively high sulphur content has been consumed. 